Earth rotates
on its axis once each day and orbits the Sun once each year. Earth
orbits at an average distance from the Sun of 1 AU and with an axis
tilt of 23 to a line perpendicular to the ecliptic plane.

As Earth
rotates, your speed around Earth’s axis depends on your location: The closer
you are to the equator, the faster you travel with rotation.

Notice that
Earth rotates from west to east, which is why the Sun appears to rise in the
east and set in the west.

Earth takes a year to complete an orbit of the Sun, but its orbital speed is still surprisingly fast. Notice that Earth both rotates and orbits
counterclockwise as
viewed from above the
North Pole.

How is
our solar system moving in the Milky Way Galaxy?

We move randomly relative to other stars in our
local solar neighborhood. The speeds are substantial by earthly standards, but
stars are so far away that their motion is undetectable to the naked eye. Our
Sun and other stars in our neighborhood orbit the center of the galaxy every
230 million years, because the entire galaxy is rotating.

Our Local Solar Neighborhood

The small box shows that stars within the local
solar neighborhood (like the stars of any other small region of the galaxy)
move essentially at random relative to one another. They also generally move
quite fast.

Galactic Rotation

Our solar system, located about 27,000
light-years from the galactic center, completes one orbit of the galaxy in
about 230 million years. Even if you could watch from outside our galaxy, this
motion would be unnoticeable to your naked eye. However, if you calculate the
speed of our solar system as we orbit the center of the galaxy, you will find
that it is close to 800,000 kilometers per hour (500,000 miles per hour).

Stars at different distances from the galactic
center orbit at different speeds, and we can learn how mass is distributed in
the galaxy by measuring these speeds. Such studies indicate that the stars in
the disk of the galaxy represent only the “tip of the iceberg” compared to the
mass of the entire galaxy.

Most of the mass of the galaxy seems to be
located outside the visible disk, in what we call the halo. We don’t
know the nature of this mass, but we call it dark matter because we have
not detected any light coming from it.

Studies of other galaxies suggest that they
also are made mostly of dark matter, which means this mysterious matter must
significantly outweigh the ordinary matter that makes up planets and stars. An
even more mysterious dark energy seems to make up much of the total
energy content of the universe.

This painting shows an edge-on view of the Milky Way Galaxy. Study of galactic rotation shows that although most visible stars lie in the disk and central
bulge, most of the mass
lies in the halo that surrounds and encompasses the disk. Because this mass emits no light that we have
detected, we call it dark matter.

How do galaxies move within the universe?

Galaxies move essentially at random within the
Local Group, but all galaxies beyond the Local Group are moving away from us.
More distant galaxies are moving faster, which tells us that we live in an
expanding universe.

Two small galaxies (known as the Large and
Small Magellanic Clouds) apparently orbit our Milky Way Galaxy.

For example, the Milky Way is moving toward the
Andromeda Galaxy at about 300,000 kilometers per hour (180,000 miles per hour).
Despite this high speed, we needn’t worry about a collision anytime soon. Even
if the Milky Way and Andromeda Galaxies are approaching each other head-on, it
will be billions of years before any collision begins.

When we look outside the Local Group, however,
we find two astonishing facts recognized in the 1920s by Edwin Hubble, for whom
the Hubble Space Telescope was named:

1)Virtually every galaxy outside the
Local Group is moving away from us.

2)The more distant the galaxy, the
faster it appears to be racing away.

Natural explanation: The entire
universe is expanding.

Are we ever sitting still?

We are never truly sitting still. We spin
around Earth’s axis and orbit the Sun. Our solar system moves among the stars
of the local solar neighborhood while orbiting the center of the Milky Way
Galaxy. Our galaxy moves among the other galaxies of the Local Group, while all
other galaxies move away from us in our expanding universe.

On a scale of 1 to 10
billion, the Sun is about the size of a grapefruit. Planets are much smaller,
with Earth the size of a ball point and Jupiter the size of a marble on this
scale. The distances between planets are huge compared to their sizes, with
Earth orbiting 15 meters from the Sun on this scale.

Seeing our solar
system to scale also helps put space exploration into perspective. The Moon,
the only other world on which humans have ever stepped, lies only about 4
centimeters (1 inches) from Earth in the Voyage model.

The trip to Mars is
some 200 times as far as the trip to the Moon, even when Mars is on the same
side of its orbit as Earth.

How far away are the stars?

On the
1-to-10-billion scale, it is possible to walk from the Sunto Pluto in
just a few minutes. On the same scale, the nearest stars besides the Sun are
thousands of kilometers away.

The nearest star
system to our own, a three-star system called Alpha Centauri, is about 4.4
light-years away.

How big is the Milky Way?

Using a scale on
which the Milky Way galaxy is the size of a football field, the distance to the
nearest star would be only about 4 millimeters. There are so many stars in our
galaxy that it would take thousands of years just to count them.

The observable
universe contains roughly 100 billion galaxies, and the total number of stars
is comparable to the number of grains of dry sand on all the beaches on Earth.

How do our lifetimes compare to the age of the
universe?

On a cosmic calendar
that compresses the history of the universe into 1 year, human civilization is
just a few seconds old, and a human lifetime lasts only a fraction of a second.

The cosmic calendar compresses the 14 billion-year history of the universe into 1 year,
so that each month represents a little more than 1 billion years. This cosmic
calendar is adapted from a version created by
Carl Sagan.

One
light-year (ly) is defined as the distance that light can
travel in 1 year. This distance is fixed because light always
travels at the same speed—the speed
of light, which is 300,000 km/s
(186,000 mi/s).

We can
calculate the distance represented by a lightyear by recalling that

Distance =
speed x time

For example,
if you travel at a speed of 50 km/hr for 2 hours,
you will travel 100 km. To find the distance represented by 1
light-year, we simply multiply the speed of light
by 1 year:

1 light-year =
(speed of light) x (1 yr)

That is,
1 light-year is equivalent to 9.46 trillion km, which is easier
to remember as almost 10 trillion km.

By studying stars of different ages, we have
learned that the early universe contained only the simplest chemical elements:
hydrogen and helium (and a trace of lithium). We and Earth are made primarily
of other elements, such as carbon, nitrogen, oxygen, and iron. Where did
these other elements come from? Evidence shows that these elements were
manufactured by stars—some through the nuclear fusion that makes stars
shine, and others through nuclear reactions accompanying the explosions
that end stellar lives.

By the time our solar system formed, about 4
billion years ago, earlier generations of stars had converted about 2% of our
galaxy’s original hydrogen and helium into heavier elements. Therefore, the
cloud that gave birth to our solar system was made of about 98% hydrogen and
helium and 2% other elements. That 2% may seem a small amount, but it was more
than enough to make the small rocky planets of our solar system, including
Earth. On Earth, some of these elements became the raw ingredients of simple
life forms, which ultimately blossomed into the great diversity of life on
Earth today.

How can we know that the universe was like in
the past?

Light takes time to travel through space, so
the farther away we look in distance, the further back we look in time. When we
look billions of light-years away, we see pieces of the universe as they
were billions of years ago.

Because light takes time to travel through
space, we are led to a remarkable fact: The farther away we look in
distance, the further back we look in time. For example, the brightest
star in the night sky, Sirius, is about 8 light-years away, which means its
light takes about 8 years to reach us. When we look at Sirius, we are seeing it
not as it is today but as it was about 8 years ago.

The Andromeda Galaxy (also known as M31) lies
about 2.5 million light-years from Earth. Figure 1.3 is therefore a picture of
how this galaxy looked about 2.5 million years ago, when early humans were
first walking on Earth.

Can we see the entire universe?

No. The age of the universe limits the extent
of our observable universe. Because the universe is about 14 billion
years old, our observable universe extends to a distance of about 14 billion
light-years. If we tried to look beyond that distance, we’d be trying to look
to a time before the universe existed.

Star - A large, glowing ball of gas that
generates heat and light through nuclear fusion in its core. Our Sun is a star.

Planet -
A moderately large object
that orbits a star and shines primarily by reflecting light from its star.
According to a definition approved in 2006, an object can be considered a
planet only if it (1) orbits a star; (2) is large enough for its own gravity to
make it round; and (3) has cleared most other objects from its orbital path. An
object that meets the first two criteria but has not cleared its orbital
path, like Pluto, is designated a dwarf planet.

Moon (or
satellite) - An
object that orbits a planet. The term satellite can refer to any object
orbiting another object.

Asteroid
- A relatively
small and rocky object that orbits a star.

Comet - A relatively small and ice-rich
object that orbits a star.

Collections of Astronomical Objects

Solar
system - The Sun
and all the material that orbits it, including the planets, dwarf planets, and
small solar system bodies. Although the term solar system technically
refers only to our own star system (solar means “of the Sun”), it is
often applied to other star systems as well.

Star
system - A star
(sometimes more than one star) and any planets and other materials that orbit
it.

Galaxy -
A great island of
stars in space, containing from a few hundred million to a trillion or more
stars, all held together by gravity and orbiting a common center.

Cluster
(or group) of galaxies - A collection of galaxies bound together by gravity. Small collections
(up to a few dozen galaxies) are generally called groups, while larger
collections are called clusters.

Supercluster
- A gigantic region
of space where many individual galaxies and many groups and clusters of
galaxies are packed more closely together than elsewhere in the universe.

Universe
(or cosmos) - The
sum total of all matter and energy—that is, all galaxies and everything between
them.

Observable
universe - The
portion of the entire universe that can be seen from Earth, at least in
principle. The observable universe is probably only a tiny portion of the
entire universe.

Astronomical Distance Units

Astronomical
unit (AU) - The
average distance between Earth and the Sun, which is about 150 million
kilometers. More technically, 1 AU is the length of the semimajor axis of
Earth’s orbit.

Light-year
- The distance that
light can travel in 1 year, which is about 9.46 trillion kilometers.

Terms Relating to Motion

Rotation
- The spinning of
an object around its axis. For example, Earth rotates once each day around its
axis, which is an imaginary line connecting the North Pole to the South Pole.

Orbit
(revolution) - The
orbital motion of one object around another. For example, Earth orbits around
the Sun once each year.

Expansion
(of the universe) - The
increase in the average distance between galaxies as time progresses. Note that
while the universe as a whole is expanding, individual galaxies and galaxy
clusters do not expand.

Earth is a
planet orbiting the Sun. Our Sun is one of more than 100 billion stars in the Milky
Way Galaxy. Our galaxy is one of about 40 galaxies in the Local Group.
The Local Group is one small part of the LocalSupercluster,
which is one small part of the universe.

Billions of
other galaxies are scattered throughout space. Some galaxies are fairly
isolated, but many others are found in groups.

Our Milky
Way, for example, is one of the two largest among about 40 galaxies in the Local
Group. Groups of galaxies with more than a few dozen members are often called
galaxy clusters.

On a very
large scale, observations show that galaxies and galaxy clusters appear to be
arranged in giant chains and sheets with huge voids between them.

The regions
in which galaxies and galaxy clusters are most tightly packed are called superclusters,
which are essentially clusters of galaxy clusters.

Our Local
Group is located in the outskirts of the Local Supercluster.

Together,
all these structures make up our universe. In other words, the universe
is the sum total of all matter and energy, encompassing the superclusters and
voids and everything within them.

How did
we come to be?

The
universe began in the Big Bang and has been expanding ever since,except
in localized regions where gravityhas caused matter to collapse intogalaxies and stars. The Big Bang essentiallyproduced only two
chemicalelements: hydrogen and helium. Therest have been
produced by stars,which is why we are “star stuff.”

The Big
Bang and the Expanding Universe

Telescopic
observations of distant galaxies show that the entire universe is expanding,
meaning that the average distances between galaxies are increasing with
time. This fact implies that galaxies must have been closer together in the
past, and if we go back far enough, we must reach the point at which the
expansion began. We call this beginning the Big Bang, and from the
observed rate of expansion we estimate that it occurred about 14 billion years
ago.

The
universe as a whole has continued to expand ever since the Big Bang, but on
smaller scales the force of gravity has drawn matter together. Structures such
as galaxies and galaxy clusters occupy regions where gravity has won out
against the overall expansion. That is, while the universe as a whole continues
to expand, individual galaxies and galaxy clusters do not expand.

Stellar Lives and Galactic Recycling

Within
galaxies like the Milky Way, gravity drives the collapse of clouds of gas and
dust to form stars and planets. Stars are not living organisms, but they
nonetheless go through “life cycles.” A star is born when gravity compresses
the material in a cloud to the point where the center becomes dense and hot
enough to generate energy by nuclear fusion, the process in which
lightweight atomic nuclei smash together and stick (or fuse) to make heavier
nuclei.

The star
“lives” as long as it can generate energy from fusion and “dies” when it
exhausts its usable fuel.

In its
final death throes, a star blows much of its content back out into space. In
particular, massive stars die in titanic explosions called supernovae.
The returned matter mixes with other matter floating between the stars in the
galaxy, eventually becoming part of new clouds of gas and dust from which
future generations of stars can be born. Galaxies therefore function as cosmic
recycling plants, recycling material expelled from dying stars into new
generations of stars and planets. Our own solar system is a product of many
generations of such recycling.